An amplifier 1 amplifies a video signal Vi of an object tramsmitted from a charge coupled device(CCD) (not shown). Separators 2,3,4 separate the video signal into a luminance signal(Y signal), a red signal(R signal), a blue signal(B signal) respectively. Gain Control amplifiers 5,6 amplify the separated R signal and the separated B signal respectively. Substractors 7,8 substract the Y signal from the R signal from and the B signal respectively generate an R-Y signal and a B-Y signal respectively. An encoder 9 generates an encoded signal E1 according to the R-Y signal and the B-Y signal. A mixer 10 mixes the Y signal and the encoded signal E1 to generate a mixed signal Vout.
On the other hand, integrators 11, 12 integrate the R-Y signal and the B-Y signal transmitted from the substractors 7,8 respectively to generate smoothened signals S1, S2 respectively. Comparators 13, 14 compare the smoothened signals S1, S2 with predetermined reference voltages Vr1, Vr2 respectively to control the gains of the gain control amplifiers 5,6 respectively. These are performed in real time. Accordingly, the white balance of the mixed signal Vout generated in the mixer 10 is controlled.
However, the conventional white balance controlling circuit has such a problem that it reproduces colors quite different with the natural color as the conventional circuit tends to shift to white when the image signal is monochromic or most of the region of the image signal is monochromatic, and also when the level of the luminance is high.